[Proposal] Let BelongsToMany->sync return affected IDs 

I think a useful feature would be if the sync method somehow returned the IDs of the relationships affected by the sync.

The only way I could think of would be to return an array containing sub-arrays like this
```php
array(
    // The IDs that were newly attached because of the sync
    'attached' => array(1,2,3),

    // The IDs that were detached during the sync
    'detached' => array(4,5),

    // The synced IDs that already existed, but had attributes, 
    // thus being updated
    'updated'  => array(6,7)
);
```
This solution feels a little clumsy, and not that elegant, but it's the simplest solution I've come to think of yet.